A glow lamp typically is comprised of a light transmitting envelope containing a noble gas and mercury with a phosphor coating on an inner surface of the envelope which is adapted to admit visible light upon absorption of ultraviolet radiation that occurs when the lamp is excited. The lamp is excited by means of the application of a voltage between the lamp electrodes. Current flows between the electrodes after a certain potential is applied to the electrodes, commonly referred to as the breakdown voltage. An elementary explanation of the phenomenon is that the gas between the electrodes becomes ionized at a certain voltage, conducts current, and emits ultraviolet radiation. Examples of typical glow discharge lamps are found in U.S. Pat. Nos. 2,067,129 to Marden; 3,814,971 to Bhattacharya; and 4,408,141 to Byszewski et al.
Emissive electrodes are utilized in fluorescent lamps to supply free electrons, thereby enabling current flow in the fluorescent tube and have, therefore, been referred to as cathodes.
The cathodes normally comprise one or more of the alkaline earth metals and compounds thereof, as these materials have relatively low work functions and are therefore able to supply free electrons without requiring the expenditure of great amounts of energy. The provisions of these free electrons by the emissive alkaline earth materials consume the electrode material and when the material is depleted to the point where it can no longer supply sufficient electrons for lamp operation upon the application of standard fluorescent lamp voltages, the lamp will fail and will have to be discarded.
Cathodes of the type well known in the art are normally made by painting, dipping or otherwise adhering a co-precipitated triple carbonate suspension, usually comprising strontium carbonate, calcium carbonate, and barium carbonate to a coil tungsten wire. The emissive materials are adhered to the coil substrate by temporary adhesive binders such as cellulose nitrate. During the activation or breakdown process, the binder is removed by thermal decomposition and the cathode is subsequently heated to a sufficiently high temperature to decompose the carbonates to their respective oxides. Typically, about 6 milligrams (for a 40-watt coil) to 30 milligrams (for a VHO coil) of the electron emissive material is coated onto the surface of the tungsten filament. It is well known in the art that during cathode activation and lamp operation, excess barium can diffuse through the emissive coating mass and subsequently evaporate from the coating surface. This evaporation of barium not only results in a darkening of the adjacent phosphor coating causing a depreciation of the light output of the lamp, but also results in a reduced lamp life owing to the depletion of barium necessary for the thermionic emission of electrons. The quantity of this commonly used electron emissive alkaline earth material is limited by the aforementioned tungsten coil.
Many attempts have been made in an effort to prevent these undesirable effects in fluorescent lamps. For example, U.S. Pat. No. 2,657,325, which issued to Homer et al on Oct. 27, 1953, teaches the provision of a monomolecular layer of tungsten oxide at the interface between the tungsten and the alkaline earth oxide. The monomolecular layer is applied by directly oxidizing the tungsten before applying the carbonate coating. The lamp is thereafter exhausted rapidly under vacuum while the alkaline earth carbonates are broken down to the alkaline earth oxides by heating. The heating is done rapidly so as to prevent any further oxidation of the metal by the carbon dioxide given off. Alternatively, the wire can be initially free from appreciable oxidation, and can be oxidized by allowing the carbon dioxide produced during breakdown of the carbonates, to oxidize the tungsten or other metal sufficiently, and be itself reduced.
U.S. Pat. No. 3,837,909, which issued to Menelly on Sept. 24, 1974, teaches a method of increasing the life of a fluorescent lamp by embodying substantially more emissive material than coated coil electrodes commonly known in the art. In the Menelly patent, an inert gas (non-oxidizing), such as, argon at a temperature of approximately 500 degrees Celsius is directed toward the coil after being coated with a plastic-like coating of barium peroxide and cellulose nitrate. When the temperature of the coil and plastic-like coating reaches a temperature of approximately 400 degrees to 500 degrees Celsius, the barium peroxide begins to melt and flow over the tungsten coil substrate. As the temperature of the coil and coating increases to approximately 500 degrees Celsius, the cellulose nitrate binder reacts exothermally, decomposing into gases, the majority gas being nitric oxide, which are expunged from the mixture. The exothermal reaction of the cellulose nitrate raises the temperature of the coil to approximately 700 degrees Celsius. The barium peroxide reacts exothermally, causing the barium peroxide to decompose into barium oxide and to simultaneously release its excess oxygen before the coating mass eventually begins to solidify. Menelly teaches minimizing any oxidation of the coil substrate by flushing away the oxides by the argon stream.
In both of the aforementioned patents, oxidation produced after the coil has been coated relies on the oxygen released during coil activation.
One method of increasing the life of a fluorescent glow lamp is described in copending U.S. Ser. No. 139,397 which was filed on Dec. 30, 1987 and which is assigned to the same Assignee as the present Application. This method involves the use of a DC mode of operation coupled with the use of a anode devoid of emissive material.
Although the above-described methods may increase the life of a fluorescent lamp to some degree, it is desirable to have more improved alternative methods.